One way to identify bacteria is through gram staining. Gram-positive bacteria are bacteria that can retain the crystal violet gram stain appearing a dark blue/purple. Gram-negative bacteria cannot retain the crystal stain and absorb the counterstain which appears red/pink.
In the image above on the left, most bacteria present are gram-positive, while there are a few gram-negative — evident by the stain color of the bacteria. While, the right picture in completely gram-negative bacteria.